This invention relates to a disc cartridge, and particularly to a shutter to be provided to open or close head insert openings of the cartridge case.
It has been known that a cartridge housing a disc-type recording medium such as a magnetic disc or an optical disc is engaged with a disc driving device, and subjected to recording and reproduction of information, while housing the disc-type recording medium in the disc cartridge in order to protect the disc-type recording medium from foreign matter such as dust, etc. and simplify the handling of the disc-type recording medium.
In such a disc cartridge, head insert openings are provided for making a recording-reproduction head provided in the disc driving device accessible to the disc-type recording medium in the cartridge case housing the disc-type recording medium in a turnable state. So long as the head insert openings are always left open, dust is liable to enter the cartridge case when not used, or fingers, etc. are liable to come into contact with the recording surface of the disc-type recording medium when handling the disc cartridge, thereby fouling or damaging the recording surface and deteriorating the recording and reproduction. Thus, a shutter for closing the head insert openings, when the disc cartridge is not in use, is usually provided in a slidable state in such a disc cartridge.
As the shutter, a U-shaped shutter made from a thin metal sheet, for example, a thin stainless steel sheet, has been often used owing to the requirements for a smaller shutter thickness, but the metal shutter has a high hardness and scrapes the cartridge case when repeatedly slid over the cartridge case, producing attrition powder, which deposits on the recording medium or the head, causing various troubles. Furthermore, the metal shutter is press-molded and thus does not have too high of a dimensional accuracy. Furthermore, the metal shutter has a large weight and thus the force of inertia leads to such trouble as shutter disengagement or overrun.
In addition to good compatibility with the cartridge case and less attrition of the cartridge case even if slid repeatedly, the following are some of the characteristics required for the shutter:
(1) The resistance to the surrounding atmosphere must be high and changes in the shape with time must be small;
(2) The toughness must be so high that the shutter must not be easily deformed or damaged;
(3) The surface must resist damage and must have a good appearance; and
(4) The moldability must be so high so as to meet mass production.
Heretofore, extensive studies have been made of shutter resin materials that can meet these requirements, but no resin material that can satisfy all of these requirements has been found yet. That is, shutter resin materials that can meet some of these requirements have been found. For example, in order to prevent the attrition of the cartridge case, polyacetal resin has been selected as a shutter resin material having a distinguished slidability and good compatibility with the cartridge case. However, the polyacetal resin had a high water absorbability and thus the shutter molded from the polyacetal resin had a poor resistance to the surrounding atmosphere, for example, it was readily deformed at a high temperature and a high humidity such as 45.degree. C. and 90% RH. Thus, the thin closing cover parts of the shutter for closing the head insert openings expanded, deteriorating the appearance or making engagement or disengagement with or from the disc driving device difficult.
In order to increase the bending modulus of elasticity of the shutter and also increase the surface hardness, thereby making the shutter surface less susceptible to damage, it was proposed to use propylene resin containing a specific amount of a filler as a shutter resin material. However, the proposed shutter resin material had a low elastic limit and the shutter molded therefrom undergoes plastic deformation when subjected to a forced displacement.